In the past, rhodium complexes of the ditertiary phosphine of the formula: ##STR1## where R is tertiarybutoxy, have been used as catalysts in the enantioselective hydrogenation of .alpha.-keto-.beta.,.beta.-dimethyl-.gamma.-butyrolactone which has the formula: ##STR2## to R-(-)-pantolactone which has the formula: ##STR3## See Achiwa et al., Tetrahedron Letters, No. 50, 4431 (1977); Ojima, J. Org. Chem., 43, 3444 (1978); and Achiwa, Chem. Lett. 297 (1978).
For certain uses of the compound of formula IV as a pharmaceutical, it is necessary to produce this compound with the correct epimeric configuration of the chiral carbon atom. Therefore, it is necessary to use a catalyst which effects a highly enantioselective hydrogenation of the ketolactone precursor of formula III. It is also desired to find a catalyst that will carry out this hydrogenation quickly and efficiently. Since the rhodium complex catalyst is relatively expensive, it has been desired to find a hydrogenation catalyst that will not be destroyed during the reaction and can easily be recovered from the reaction medium. It is these properties which will provide a catalyst that can be reused for many hydrogenation procedures.
While pantolactone has been obtained in optically active form by resolution, the catalytic asymmetric synthesis has proven to be more effective from a commercial standpoint. To optimize catalytic assymetric synthesis, there has been a need for catalysts which can provide high yields, fast rates and use low catalyst loadings.